The roof of many large crude storage tanks is open in the sense that there is no permanently attached roof. The roof floats on top of the product. To prevent vapors from escaping from around the edge of the roof, it is common to provide some sort of seal. These seals are made of a non-conductive material, usually neoprene. This material isolates the roof from the tank wall electrically and from any connection to earth. To overcome this problem, the industry has installed a device called a “shunt.” These shunts are attached to the roof in such a manner that they are to be in constant contact with the tank regardless of the position of the floating roof. To make contact, these shunts are made with metal fingers which are spring-loaded and are made springy by the material used. These shunts require constant maintenance for several reasons:
1. Since the roof floats, it can easily drift slightly off-center and disconnect from the shunt on the opposite side.
2. Wax and other heavy crude elements tend to deposit between the tank wall and the shunt fingers, leaving a small gap which can cause arcing and fire.
Such a fire is called a “rim fire”. In this case, the arc ignites the gasses always prevalent around the rim just above the seal.
During an electrical storm, the electrostatic field will induce a charge on both the tank and the floating roof. The present invention (RGA) will equalize the electrical potential between the tank shell (wall) and floating roof at all times, but especially in the case of a direct lightning strike to either the floating roof itself or tank shell. Charge from the direct (or nearby) strike will simultaneously flow in all directions across the shell and roof, eventually reaching ground via the shell. As charge on the roof attempts to reach ground, arcing will be produced at the shunts. If the shunts are not in perfect contact with the tank wall, the charge will create an arc when that storm cell is discharged by a direct or nearby strike. In accordance with testing performed by Cullham on behalf of the API 545 committee, the fast component of the lightning strike waveform is too short to ignite vapors. Rather, it is the rather longer follow-on current that may produce arcing and sparking which can ignite the vapors.
Eliminating the risk of arcing requires making a full-time positive connection between the tank wall and roof of the tank, as well as having a low resistance path of about 30 milli-ohm or less. Making a positive connection will eliminate this risk and any other phenomena that can create dangerous arcing conditions. The present invention discloses a grounding system that improves the prior art by resisting corrosion using an aluminum cable instead of a tinned copper cable and reduce resistance with an improved reel assembly.